MY2000 Impreza EJ251 ECU Running a Turbocharged EJ205

The Subaru Impreza 2.5RS sedan and coupe have been a popular platform for turbo swaps for a while now. Typically the donor turbo engine was not compatible with the factory EJ251 ECU and required a complicated wiring harness merge to get the car running on a swapped-in turbo ECU. Here you can see how it can be done without the wiring hassle by keeping the EJ251 ECU and using our projectLAMBDA Tuner to get it to play nice with the turbo engine. With the engine harness connectors being nearly the same, the swap becomes almost a plug and play affair.

For this build, an EJ205 from a JDM Forester SF5 was swapped in to replace the original EJ251. Other suitable candidates are the EJ207 and EJ205 from the Impreza GC8 and GF8 WRX/STI. A lot of the sensors and wiring are shared between these 2 engines despite the fact that one is turbocharged and the other is not. Both come with a 32-bit ECU although the JDM runs on a mass airflow sensor, has different harness connectors and does not support OBD2 protocol. The USDM NA ECU uses a speed density fuel calculation instead of MAF and also has a Bosch LSU 4.2 wideband front O2 (lambda) sensor from the factory so that needs to be taken into account when doing the swap. The USDM ECU also does not have boost control but we are going to change that in the future. For now a manual boost controller was installed to keep the turbo under control.

The original EJ251 intake manifold has a Bosch 1.0 bar TMAP installed in the plenum to measure manifold pressure and air temperature. Since the EJ205 does not have any provision for this, a GM intake air temperature sensor was installed to support the ECU’s speed-density fueling. A 3.0 bar MAP sensor measures boost off one of the intake manifold vacuum ports. Another cleaner option would be to modify the manifold to fit a Bosch Motorsport 3.0 bar TMAP. This gives minimal measurement delay on the MAP signal versus a remotely mounted MAP sensor.

The owner stepped up from the original TD04HL to a modified VF39, then finally an FP Blue along with supporting mods to reach his power goals. In its current form, the engine is tuned for up to 24 psi of boost on 93 octane pump gas, but 22 psi seemed to be the sweet spot for power and torque. This could be a limitation of the unmodified EJ205 heads and cams. Not bad at all considering the JDM EJ205 tune has a boost target of only 10.6 psi. The end result is a cool-looking daily driver that can back up its looks with performance.

Build Info

• MY2000 Impreza 2.5RS sedan chassis
• JDM EJ205, stock block and heads, 9.0:1 CR
• FP Blue Turbo with EWG, 1.0 bar wastegate spring
• DW1000 Injectors
• VA STI top mount intercooler
• Header, downpipe, exhaust

Configuring injector settings is one of the first things to do when setting up your base map. The two main parameters for configuration injectors are ‘Load Scalar’ and ‘Airflow Scalar’. These should also be updated if the engine’s size (displacement) has been changed or you’re running a fuel other than straight gasoline.

For easy reference, I’ve done the calculations for some commonly used injectors so that you don’t have to. Just copy and paste the numbers into your current tune.

These load and airflow scalars were calculated a 2.5L engine running on gasoline. If you’ve got a different engine displacement or are running a fuel other than gasoline (such as E85), see the parameter explanations after the table.

						Latency	Latency
Top Feed Injector	Part Number	Color	Flow Rate (cc)	Load Scalar	Airflow Scalar	Slope (ms/V)	Offset (ms)
EJ251	16611AA430	Grey	280	0.0723	0.9621	0.10	1.76
EJ205	16611AA521	Light blue	420	0.0482	1.4432	0.12	2.36
EJ207	16611AA510	Pink	550	0.0368	1.8899	0.12	2.36
EJ255/EJ257	16611AA720	Dark blue	550	0.0368	1.8899	0.13	2.62
						Latency	Latency
Side Feed Injector	Part Number	Color	Flow Rate (cc)	Load Scalar	Airflow Scalar	Slope (ms/V)	Offset (ms)
EJ205/EJ207	16611AA370	Yellow	500	0.0405	1.7181	0.11	2.03
EJ255/EJ257	16600AA170	Yellow	500	0.0405	1.7181	0.11	2.14

Note: Flow rates are estimated based on ECU data. Use flow test data when available.

 

Load and Airflow Scalars

These two parameters can be found under ‘Configuration’ in the map tree. The ‘load scalar’ is a combination of injector flow rate, engine displacement and stoichiometric ratio all in one parameter.  It is used to calculate how much fuel is needed based on the volumetric efficiency map. ‘Airflow scalar’ is used to calculate the mass airflow of the engine from engine load and engine speed (RPM).

How to calculate what your new load and airflow scalars should be:

For an injector change:

NEW LOAD SCALAR = OLD LOAD SCALAR x (OLD INJECTOR SIZE / NEW INJECTOR SIZE)

NEW AIRFLOW SCALAR = OLD AIRFLOW SCALAR x (NEW INJECTOR SIZE / OLD INJECTOR SIZE)

For an engine displacement change:

NEW LOAD SCALAR = OLD LOAD SCALAR x (NEW DISPLACEMENT / OLD DISPLACEMENT)

For a stoichiometric ratio change:

NEW LOAD SCALAR = OLD LOAD SCALAR x (OLD STOICH RATIO / NEW STOICH RATIO)

NEW AIRFLOW SCALAR = OLD AIRFLOW SCALAR x (NEW STOICH RATIO / OLD STOICH RATIO)

For multiple changes you can chain the equations together in any order to get the final value.

 

Injector Latency

Fuel injectors don’t open instantly. There is a lag time between when the ‘on’ signal is sent and when the fuel injector fully opens. This is called ‘Injector Latency’. This latency changes with voltage. At lower voltages, the injector will take longer to open. At higher voltage, it opens faster.

To compensate for injector latency and inject the correct amount of fuel, the ECU adds latency to the fuel injector pulse width.

If you’ve swapped injectors you will need to use the latency values for the new injectors. Incorrect latency settings can cause fueling consistency problems. This is most noticeable at idle and low engine load.

The following equation describes how the ECU calculates injector latency:

LATENCY = OFFSET - (SLOPE x VOLTAGE)

‘Slope’ and ‘Offset’ are two parameters which can be modified by re-flashing. They are found under the ‘Fuel Injectors’ group in the map tree. Slope determines how quickly the latency falls with increasing voltage. Offset is the starting point from which the slope subtracts.

 

Cranking Base Injector Pulse Width

This map is found under the ‘Fuel – Base’ group. It sets the injector pulse width when starting the engine. Scale the entire map down when using larger injectors. Do the opposite for smaller injectors. These values already include injector latency, so you may want to take that into account when making changes. Once the engine gets to about 400 rpm the ECU will stop using this map and switch over to regular pulse width calculations.

 

Minimum Injector Pulse Width

Also found under the ‘Fuel Injectors’ group. This limits the minimum pulse width that the ECU is allowed to use. When pulse width times get very low, the injector doesn’t fully open and the flow rate won’t be accurate. If you’ve got larger injectors you may find reducing this helps solve rich conditions at low load and idle.